1. Field of the Invention
The present invention relates to a lateral transient voltage suppressor, particularly to a lateral transient voltage suppressor for low-voltage applications.
2. Description of the Related Art
Because the IC device sizes have been shrunk to nanometer scale, the consumer electronics, like the laptop and mobile devices, have been designed to be much smaller than ever. Without suitable protection devices, the functions of these electronics could be reset or even damaged under ESD (Electrostatic Discharge) events. Currently, all consumer electronics are expected to pass the ESD test requirement of IEC 61000-4-2 standard. TVS (Transient Voltage Suppressor) is generally designed to bypass the ESD energy, so that the electronic systems can be prevented from ESD damages. The working principle of TVS is shown in FIG. 1. In FIG. 1, the TVS devices 10 are connected in parallel with the protected circuits 12 on the PCB (Printed Circuit Board). These TVS devices 10 would be triggered immediately when the ESD event is occurred. In that way, each TVS device 10 can provide a superiorly low resistance path for discharging the transient ESD current, so that the energy of the ESD transient current can be bypassed by the TVS devices 10.
For advanced interface applications, such as Ethernet, LVDS . . . , etc, the signal swing or power supply voltage is lower than 2.5V. FIG. 2 shows the schematic of tradition TVS with zener diode 14. The turned-on voltage of TVS in FIG. 2 is decided by the breakdown voltage of zener diode 14. However, the traditional zener breakdown voltage of TVS is around 6V˜10V, which is higher than the breakdown voltage of the device to be protected. When ESD event is occurred, the device to be protected will breakdown first and damaged before TVS turned-on. Therefore, the turned-on voltage of TVS has to be reduced for under 2.5V application.
The traditional method to reduce breakdown voltage is adjusted the p-n junction dosage of zener diode 14. However, the leakage current will be huge if the breakdown voltage is adjusted to around 3V because the very heavy dosage of p-n junction. FIG. 3 shows another method that diodes 16 in series are used to instead of zener diode between Vcc and GND. The forward cut-in voltage of one diode 16 is around 0.7V. In ideal the turned-on voltage of TVS is n*0.7V when n diodes 16 in series. However, from FIG. 4 and FIG. 5, it is known that the more the number of the transistors cascaded to each other, the larger the leakage current due to the parasitic PNP formed by P-type substrate 20 and each N-type doped well 22. This phenomenon is the Darlington effect. The turned-on voltage of n diodes 16 in series is much lower than n*0.7V and leakage current is huge in CMOS process because the Darlington effect in FIG. 3.
To overcome the abovementioned problems, the present invention provides a lateral transient voltage suppressor for low-voltage applications, so as to solve the afore-mentioned problems of the prior art.